This invention relates, in general, to micro electro-mechanical system (MEMS) devices and, more particularly, to a high quality (high-Q) variable capacitor fabricated using MEMS technology.
One prior art type of variable capacitor, known as the thermal drive variable capacitor 10, is illustrated in FIG. 1. In this prior art version, a dielectric gap 11 between two capacitor plates 12 and 13 is controlled or altered by means of thermal expansion of lateral components. As shown in FIG. 1, each end of an upper plate 12 of a capacitor is connected to one end of a movable, hinged diagonal component 15. The other end of the diagonal component is connected to a lateral component 14. Movement of lateral component 14, which is controlled by thermal devices, causes the hinged diagonal components 15 to translate the lateral movement to vertical movement of upper plate 12. The vertical movement of upper plate 12 varies the capacitance of the device.
The drawbacks to thermal drive variable capacitor 10 are several. The mechanical design of the device is complicated resulting in a costly and inefficient manufacturing process. Also, the complicated design of the thermal drive negatively impacts the reliability of the device. The translation of lateral movement to vertical movement intrinsic to thermal drive devices has the negative effect of increasing the size of the device. Also, the thermal expansion and contraction operation of the device is inefficient, thus resulting in slow speed in varying the capacitance. Furthermore, thermal operation requires significantly more power consumption than electrostatically driven MEMS devices of similar capability.
Another prior art variable capacitor 20 is illustrated in FIG. 2. MEMS variable capacitor 20 has at least one driver 21, itself a simple variable capacitor, for determining the displacement of a dielectric membrane 22 and a variable capacitor region for employment with an external circuit (not shown). The displacement of dielectric membrane 22 is determined by the application of a voltage potential across drivers 21.
The application of a voltage to the drivers causes an electrostatic attraction between the driver electrodes. This electrostatic attraction results in a downward movement of dielectric membrane 22, thereby causing a downward displacement. This reduction in the gap between the upper 23 and lower 24 capacitor plates results in a corresponding variance in capacitance.
Hence, a need exists for a high-Q capacitor that is reliable, cost efficient, and has continuous dynamic response over the full displacement of the dielectric membrane.